1. Field of the Invention
The present invention relates to an energy filter, and more particularly to an energy filter for an electron microscope.
2. Discussion of Relevant Art
Imaging energy filters for electron microscopes are known, for example, from U.S. Pat. Nos. 4,812,652; 4,740,704; 4,740,261 and 5,449,914. These energy filters always have at their exit two distinct planes, the achromatic image plane and the spectrum plane, the spacing of which is the so-called Helmholtz length and amounts to about 50 mm to 70 mm. All the beams which emerge from a common point of the object or of the diffraction image are focused in the achromatic image plane, independently of their energy. A local focusing in the achromatic image plane is thus also spoken of. In the spectrum plane, on the other hand, the beams of the same energy are focused at a point, so that a spectrum of the electron beam results in the energy-dispersive direction. Energy focusing in the spectrum plane is thus also spoken of. Either the achromatic plane or the spectrum plane can be imaged on the recording plane, in which a fluorescent screen or an electron detector is arranged, by means of a projection system, consisting of a transfer lens and at least one projection lens. An energy window for imaging can be selected with a slit diaphragm arranged in the spectrum plane and having a defined width in the energy-dispersive direction. The width of the energy window is then dependent on the application. For elastic bright field imaging and for energy loss imaging, energy bandwidths in the region of 4-25 eV are meaningful. For a parallel detection of a spectrum, it should be possible to select an energy region of 50 eV up to 500 eV.
An arrangement for the setting of different energy bandwidths in the spectrum plane is provided in U.S. Pat. No. 4,812,652, and has two slit edges which are mechanically movable relative to each other. In principle, optional slit widths can be set with this arrangement. The symmetrical and parallel motion of the slit jaws of course requires an expensive mechanism. And because of mechanical hysteresis effects, an automatically reproducible setting of the energy bandwidth is possible only by means of measurement of the slit width. Since, furthermore, the two slit edges have to be arranged in different planes for constructional reasons, shadowing effects arise at small energy bandwidths.
Alternatively, it is conceivable to provide several slit diaphragms of different slit widths in a diaphragm changer arranged in the spectrum plane. The production of corresponding slit diaphragms with fixed slit dimensions is relatively easy and correspondingly inexpensive. The variation of the energy bandwidth of course requires a mechanical changing of the slit width by means of the diaphragm changer. The choice of selectable energy bandwidths is severely restricted thereby. And, because of the required accuracy of positioning, of one .mu.m, an automation of the energy bandwidth setting is very expensive.
Transmission electron microscopes or scanning electron microscopes are already known from the U.S. Pat. Nos. 5,013,913 and 5,483,073; in them, the setting of different aperture diaphragms or illuminating field diaphragms takes place by electron-optical choice of a suitable diaphragm aperture. For this purpose, a respective diaphragm is installed in the electron microscope and has numerous round diaphragm apertures with different aperture diameters. Selection, or setting to a diaphragm with a defined diaphragm diameter, takes place by the deflection and subsequent restoring of the electron beam. In contrast to the situation at the output side of an energy filter, however, much more room for the arrangement of the deflection system is available in the beam path on the illumination side.